The present invention relates generally to a method and an apparatus for measuring the shape of an object by using an optical cutting process for a visual recognition device or the like, or more in particular to a method and an apparatus for measurement with an optical cutting beam suitable for use in an environment subject to variations in external light or in the case where the reflection characteristics on the object are irregular.
In a conventional optical cutting beam measuring apparatus for a visual recognition system, as disclosed in No. JP-A-59-197810, subtraction is effected between an image irradiated with an optical cutting beam and an original image not so irradiated, and by thus removing the effect of the background light, the optical cutting beam is stably extracted.
Specifically, according to the conventional optical cutting beam measurement method, first, the original image of an object is picked up and stored in an image memory or the like. Then, a slit light beam is projected on the object, and the original image is subtracted from the image thus obtained thereby to produce a differential image. This subtraction between images offsets the parts other than the slit light beam (optical cutting beam). That is to say, any part in the background which may be bright has no effect. In actual practice, small noises that are caused in parts other than the slit beam when the differential image is produced can be removed by an appropriate threshold process. After the optical cutting beam is separated from the background, the position of the optical cutting beam is determined by thinning or statistical means. In this method, as far as the radiation surface is uniform, the brightness change of the optical cutting beam assumes a symmetric form, thereby facilitating the determination of a central value. To the extent that the light reflection characteristics of the surface of the object are irregular such as in the case where white numerals exist on the object surface and are covered by the optical cutting beam, however, the brightness of the optical cutting beam undergoes a variation. If a central value is determined from this brightness variation, the resulting figure would be deflected toward a higher reflection factor, thereby causing a problematic deterioration of the measurement accuracy.